1. Field of the Invention
The present invention relates to semiconductor lasers, and more particularly to semiconductor lasers such as a distributed feedback laser (a DFB laser) that incorporates diffraction gratings inside thereof.
2. Description of the Prior Art
For example, various types of semiconductor lasers, such as a DFB laser or a distributed Bragg reflection laser (a DBR laser), which incorporate diffraction gratings are known. In these types of semiconductor lasers, a number of lower layers such as a cladding layer, an active layer and an optical waveguide are formed on a substrate by a first stage of crystal growth. The upper side of these lower layers are periodically etched to a predetermined depth, whereby diffraction gratings are formed. A number of upper layers, such as an optical waveguide layer and a cladding layer, are formed by regrowing crystals.
One example of a DFB laser (an oscillation wavelength of 740-1000 nm) manufactured in the manner as mentioned above is disclosed in Conference Digest on 13th IEEE International Semiconductor Laser Conference, Sep. 21-25, 1992, pp. 14 -15. In this DFB laser, an n-type Al.sub.X Ga.sub.1-X As (X=0.45) cladding layer, an undoped GaAs active layer, a p-type Al.sub.X Ga.sub.1-X As (X-0.45) carrier blocking layer, a p- type Al.sub.X Ga.sub.1-X As (X =0.3) optical waveguide layer, and an n-type GaAs light absorption layer are successively grown on an n-type GaAs substrate. The layers are etched from the light absorption layer into the carrier blocking layer using lithography and dry etching techniques, so that predetermined three-dimensional diffraction gratings are formed. A p-type Al.sub.X Ga.sub.1-x As (X=0.25) optical waveguide layer, a p-type Al.sub.X Ga.sub.1-X As (X=0.45) cladding layer, and a p-type GaAs cap layer are formed over the light absorption layer by a second stage of crystal growth.
In such a conventional semiconductor laser, aluminum (Al), which is susceptible to oxidation, is included in high proportion in the lower layers such as a carrier blocking layer or the like. These lower layers are exposed to air before crystals are regrown by etching to form diffraction gratings. For this reason, the semiconductor laser has shown a failure caused by the occurrence of crystal defects. The increase of the composition of aluminum is intended to improve the characteristics of the semiconductor laser. Generally, in conventional semiconductor lasers, the content of aluminum in the group III elements is more than 30%.
Moreover, when the oscillation wavelength is set to a shorter wavelength side, for example, below 800 nm in the range of 740-1000 nm, the composition of aluminum in each layer should be increased. However, in such a case, it becomes difficult to obtain reliability over a long period of time under high output oscillation conditions.